Hepatitis C virus (HCV) is a major global health problem, with an estimated 150-200 million people infected worldwide, including at least 5 million infected individuals within the European Union (Pawlotsky, 2004). According to the World Health Organization, 3 to 4 million new infections occur each year. The infection is often asymptomatic; however, the majority of HCV-infected individuals develop chronic infection (Hoofnagle, 2002; Lauer, 2001; and Seeff, 1995). Chronic HCV infection frequently results in serious liver disease, including fibrosis and steatosis (Chisari, 2005). About 20% of patients with chronic HCV infection develop liver cirrhosis, which progresses to hepatocellular carcinoma in 5% of the cases (Hoofnagle, 2002).
Chronic HCV infection is the leading indication for liver transplantations (Seeff, 2002). Unfortunately, liver transplantation is not a cure for hepatitis C; viral recurrence is an invariable problem and leading cause of graft loss (Brown, 2005). No vaccine protecting against HCV is available. Current therapies include administration of ribavirin and/or interferon-alpha (IFN-α), two non-specific anti-viral agents. Using a combination treatment of pegylated IFN-α and ribavirin, persistent clearance is achieved in about 50% of patients with chronic hepatitis C. However, a large number of patients have contraindications to one of the components of the combination, cannot tolerate the treatment, do not respond to IFN therapy at all or experience a relapse when administration is stopped. In addition to limited efficacy and substantial side effects such as neutropenia, haemolytic anemia and severe depression, current antiviral therapies are also characterized by high cost.
Until recently, the development of more effective therapeutics to combat HCV infection has been hampered by the lack of a cell culture system supporting HCV replication. Robust production of infectious HCV in cell culture has now been achieved using a unique HCV genome derived from the blood of a Japanese patient with fulminant hepatitis C (JFH-1) (Wakita, 2005; Lindenbach, 2005; Zhong, 2005). The ability of the JFH-1 strain of HCV to release infectious particles in cell culture (HCVcc) and the development of retroviral HCV pseudoparticles (HCVpp) (Bartosch, 2003; Hsu, 2003) have allowed studies on the mechanism of HCV entry and replication, that have led to the identification of potential therapeutic target biomolecules.
HCV is a positive strand RNA virus classified in the Hepacivitus genus, within the Flaviviridae family. Translation of the major open reading frame of the HCV genome results in the production of an approximately 3000 amino acid long polyprotein, which is cleaved co- and post-translationally by the coordinated action of cellular and viral proteases into at least 10 mature proteins, including two envelope glycoproteins (E1 and E2). HCV initiates infection by attaching to molecules or receptors on the surface of hepatocytes. Current evidence suggests that at least four host cell molecules are important for HCV entry in vitro: the tetraspanin CD81 (Pileri, 1998), the scavenger receptor class B type I (SB-RI) (Zeisel, 2007; Bartosch, 2003; Grove, 2007; Kapadia, 2007; Scarselli, 2002), Occludin (Ploss, 2009) and Claudin-1 (CLDN1), an integral membrane protein and a component of tight-junction strands (Evans, 2007). HCV glycoproteins have been reported to interact directly with CD81 and SR-BI (Cocquerel, 2006). Mutagenesis and antibody-blocking studies with tagged versions of CLDN1 suggest that the first extracellular loop is involved in interactions with HCV (Evans, 2007). However, the exact role played by each of the receptors is unclear.
Identification of these receptors or co-receptors for HCV has opened up new avenues for the development of therapeutic and prophylactic agents as drug candidates for the prevention and/or treatment of HCV infection. Thus, for example, Nicosia and coworkers have generated monoclonal antibodies against native human SR-BI that inhibit HCV E2 binding to SR-BI and efficiently block HCVcc infection of hepatoma cells in a dose-dependent manner (Catanese, 2007; WO 2006/005465). European patent application No. EP 1 256 348 discloses substances with antiviral effects (e.g., antibodies, proteins, sulphated polysaccharides and low molecule compounds) that inhibit binding of HCV E2 and CD81. International patent application WO 2007/130646 describes in vitro and cell-based assays for identifying agents that interfere with HCV interactions with Claudin-1 thereby preventing HCV infection. Since the development of novel therapeutic approaches against HCV remains a high-priority goal, these studies are encouraging as they demonstrate that agents that affect HCV entry into susceptible cells may constitute an effective and safe alternative to current HCV therapies.